1. Field of the Invention (Technical Field)
The present invention relates generally to the field of particle concentrators, and more particularly to an apparatus for pre-concentrating airborne particles to promote their detection, and specifically to a particle preconcentrator apparatus having improved reliability and durability and especially useful for the detection of compounds such as explosives, illegal drugs, other controlled substances and chemical agents. In this application, as set forth more fully later in this disclosure, the term particle is intended not to exclude vapor.
2. Background Art
The use of explosives by terrorists generates tremendous interest in explosives detection. Development of explosives vapor detectors has been especially rapid during the past few years, with a growing number of ion mobility spectrometry (IMS), electron capture detector, and chemi-luminescence-based systems appearing on the market. Current explosives detection systems are hampered, however, by a need to detect explosives residues at extremely low (in the range of parts-per-trillion, ppt) concentrations, and to do so quickly (on a per person or per item basis) and with minimal invasion of the personal privacy of airline travelers and the like. As a result, a further need has arisen for a vapor particle preconcentrator apparatus, that is, an apparatus which quickly gathers vapor particles occurring in very low concentrations, and concentrates them prior to presenting them to a detecting and/or measuring device. By pre-concentrating the target particles prior to detection, known detection methods such as IMS, electron capture, and chemi-luminescence may be employed for screening purposes despite low environmental concentrations.
One goal, for example, is to detect extremely low concentrations of chemical residues that cling to, but are readily dislodged from, the clothing and/or skin of an individual who has recently handled the chemical (or is actually concealing on his person a bulk quantity of the chemical). Low concentrations of the target particles may even occur in the air immediately about the individual's body. When an individual handles or otherwise comes in contact with a modest quantity of, for example, an explosive or narcotic substance, significant numbers of particles of the substance typically cling feebly to the person's skin and/or clothing. For a period of time thereafter, dry particles or vapors of the substance may be shed or emitted from the person, particularly if a disturbance such as a blowing wind or a jostling bump causes significant numbers of particles to dislodge from the person and become entrained or suspended in the surrounding air. A desirable apparatus for testing for the presence of a target substance on an individual is capable of detecting these "dislodged" particles, permitting detection without the need for physical contact with the individual.
Many known detection devices, however, require the presence of comparatively high concentrations of target substance particles for reliable detection. Known devices employing high-volume air flows to blow or suction particles from a person suffer from particle concentration dilution, rendering detection unreliable. Moreover, known detectors for use in testing for explosives or drugs on humans, luggage, motor vehicles, and other bulky items frequently cannot properly process the large air volumes used to collect the target particles. In such circumstances, a concentrator becomes a necessity. An effective concentrator must effectively collect extremely small quantities (i.e. highly diluted) of particles from a high volume air (or other gas) flow, and then effectively release the collected (i.e. concentrated) particles for transportation to a detector device.
Thus, a goal of particle preconcentration for detection is to efficiently collect (adsorb) as many particles as possible from a large volume, low-concentration sample, and then release (desorb) the concentrated particles to a detector device. Known methods of preconcentration to collect and release particles for detection include volume collection in a bed or filter of charcoal, and thin-foil collectors.
Volume collection involves the adsorption of the target particles in a large absorbent bed, typically a bed of charcoal. Volume collection is a reasonably efficient method of absorption due to the abundance of surface area for a given volume of charcoal bedding. Particle desorption for presentation to the detector, however, is difficult and slow in volume collection systems. A particle preconcentrator based upon volume collection techniques is commercially impracticable for purposes of explosives detection due to the size of the device and the unacceptably large amount of time required to desorb the concentrated particles.
Some known preconcentrator devices utilize adsorption upon the solid surfaces of thin foils. A thin foil preconcentrator includes a collection of thin metallic foils disposed mutually parallel within a plenum. The foils, which may incorporate ultra-thin stainless steel films, are expensive to manufacture. A high-volume discharge of air bearing the low concentration of target particles is blown through the plenum and parallel between the foils. Ostensibly, target particles impact, adhere to, and accumulate upon the confronting planar faces of the foils; desorption is then promoted by heating the foils and blowing clean air parallel between the foils to dislodge accumulated particles and transport them to a detector.
Obtaining the needed surface area for particle collection in a thin foil preconcentrator can result in a physically large unit with excessive dead space or, conversely, very closely-spaced foils. To provide adequate electrical resistance in the foils to heat them during desorption, the foils must be very thin. The high-volume air flow necessary for rapid adsorption frequently causes tearing, displacement, and destruction of the thin, fragile, foils, a problem aggravated when the foils are closely spaced, thereby impeding the discharge. Discharges sufficiently lowered to reduce foil damage, and which are desirable in the desorption stage, may not be strong enough to release accumulated particles from the foil faces. Additionally, because the sample-bearing air stream blows parallel past the foils, a boundary layer forms on the foil surfaces, impeding the impact and adhesion of particles upon the foils. If a significantly large item, e.g., handkerchief, comb, pencil, or the like, should be ingested by the preconcentrator, the foils are susceptible to severe damage and loss of collection surface area. Such ingestion is a concern with actual field applications of explosive and narcotic detection.
Another known surface collection method of preconcentration is the use of a membrane filter preconcentrator. Similar to the volume collection method, the membrane filter method is very efficient for absorption of vapors and particles because of the large surface area to volume ratio. The membrane filter, however, lacks the ability to quickly desorb the vapors and particles from the surface as does the volume collection method.
A need remains, therefore, for a preconcentrator device that functions rapidly and is durable for field use, such for use within explosives detectors within airport terminals and the like.